Conventional test methods include a hierarchy of tests including wafer probe testing, packaged part testing, and system testing. In each of these, it is customary to use a test fixture between the device under test, DUT, and the tester. The test fixture normally includes a switch matrix for connecting tester pins to DUT pins, and a collection of driver and receiver circuits, switches and relays that are commonly referred to as the “pin electronics” of the tester.
In digital systems, clock rates have increased to 1 GHz and beyond. At high frequencies it becomes desirable to place components close together, and minimize trace lengths. To this end, systems have been produced that employ bare IC chips rather than packaged parts, with the IC chips attached to the system board using flip chip bonding methods, resulting in dense systems and short trace lengths. It is similarly desirable to minimize the electrical path lengths between a system under test, SUT, and the comparator circuits that are used to verify system behavior.
Conventional testing methods provide a hierarchy of inspections and tests for testing complex systems. Automatic optical inspection, AOI, employs cameras and lighting systems to examine board assemblies for correct component placement and orientation, acceptable traces and solder joints. Automated X-ray inspection, AXI, can check solder joints that are hidden from AOI equipment. In circuit test, ICT, is often employed to measure values of discrete components, and to find open and short circuits. The fixture for ICT is typically a grid of electronic probes (or “bed of nails”) that make contact with test points designed into the board assembly. The maximum number of nodes that can be tested using ICT is currently around 7,000. Complex system boards may have 10,000 nodes and upward. Boundary scan is a test method commonly used to check that all components are present and properly oriented. Boundary scan methods add logic and control signals to each component, internal blocks are chained together, and access to internal nodes is greatly improved. After all the passive elements of the system have been verified and boundary scan has been performed, a board functional test is typically required. Normally, a test fixture is used to connect system nodes to tester nodes. With the system under power, stimulus vectors are applied while system outputs are measured for the correct values. A lot of effort is required to determine the stimulus vectors and the correct system responses, and the effort increases as system complexity increases. For some complex systems this approach is abandoned because the time and effort are judged to be too great. In these cases, less rigorous testing may be accomplished using a “hot mockup” which is a version of the end-user system, or by using system equivalency tests. These methods are weak in terms of diagnosing problems, because only system level results are accessible.
Recent test fixtures have included expensive GaAs circuits for increased switching speed and electro-optical receivers for isolation between test circuits. Despite these efforts, some systems cannot be tested at full speed and others have to compromise on test coverage to keep the test time within reasonable bounds.
Recent advances in high-density interconnection (HDI), circuits, and in flip chip bonding techniques have made it possible to add additional IC chips to a system board at a low assembly cost. Also, advances in IC chip design and implementation make it possible to include most or all of the functions of a sophisticated tester on a single IC chip, especially if the pin electronics are simplified.
Built in self test (BIST) is a methodology that has been developed for testing individual components by providing test circuits within the component, rather than connecting them to an external tester. TOB is similar in concept, except that the test target is a system board rather than a component of that board.